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Liz and Steve Alderman: When we hear politicians demonize religions, single out ethnicity, or resort to jingoistic talking points, it makes us more than cringe

We can never win as a nation by divisive rhetoric or bias against ethnic or religious groups, they say

Liz and Steve Alderman's son, Peter Alderman, was killed during the 9/11 attacks. They are the co-founders of the Peter C. Alderman Foundation, an international agency that implements programs to strengthen mental health, recovery and resilience for communities devastated by violence and armed conflict. The opinions expressed in this commentary are theirs.

(CNN)Our son Peter was murdered on 9/11. He was attending a conference at Windows on the World at the World Trade Center. He did not work in the building. He was one of the nearly 3,000 people who died 15 years ago on September 11, 2001.

Liz and Steve Alderman

No parent ever recovers from the loss of a child. We are no different. Since Peter's death at the age of 25, we have devoted the past 15 years to leaving a mark that Peter existed and that the world would be a better place because he had lived.

We have sought to pay tribute to our son, and confront our own unspeakable grief, by connecting with and supporting people who have also suffered devastating loss, no matter what their race or ethnicity, no matter where they live, and no matter what religion they practice. We learned early on that we are not alone. Suffering is universal.

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By most estimates, there are over 1 billion people on this earth who have been exposed to or have directly experienced violent conflict. Many of them are desperately trying to make sense of their new reality and trying to learn what is necessary to survive. They live in places as familiar as New York, and as far away as Juba in South Sudan, Gulu Town in Northern Uganda and the refugee camps of Jordan, Lebanon and Northern Iraq. They are just like us, struggling to understand what has happened to them and struggling to get through every day.

When we hear politicians demonize religions, single out ethnicity, or resort to jingoistic talking points, it makes us more than cringe. We recoil in utter disgust. We know how violence, particularly armed violence, makes us all feel less than human, angry and, too often, more violent.

The men who murdered our son practiced an ideology of hate. Their narrowness of vision, of a world where only their world view could be tolerated, left no room for common understanding. We must examine the values in our own leaders, knowing that a lack of tolerance begets very little but a perpetuation of similar intolerance, until we are all divided and there is no place to reclaim our common humanity.

We often think how Peter would feel about the world in which we now live. He would be proud that he was born into a country that could elect its first African-American president, a sign that we as a nation had found some form of reconciliation of its greatest divide. But we suspect he would have been horrified by the decade of warfare that ensued after his death and by those who preach intolerance and exclusion. Peter practiced inclusion and kindness, a reason 15 years after his death his friends still gather every year to honor his life.

As his parents, we feel we have a responsibility to speak out for our son's principles, to carry on his legacy on this earth. As we mark the 15th year of his death, our message to every parent and every individual is that we must practice what our son so often preached; kindness, tolerance, inclusion, acceptance and love. As a close friend of Peter's said, "If life is measured in love, then Peter's was an exemplary one. Peter was loved by so many because he himself loved."

We can never win as a nation by divisive rhetoric or dividing ethnic or religious groups. We must preach and practice social inclusion, especially when it is difficult. Otherwise, we fear that our son's death, and so many of those who were murdered that day, will have died for nothing more than perpetuating a cycle of violence for which there is no end.